As is well known in the art, active CMOS image sensors typically consist of an array of pixels. Typically, each pixel consists of a photodetector element and one or more transistors to read out a voltage representing the light sensed in the photodetector. FIG. 1a shows a typical pixel layout for an active pixel image sensor. The pixel consists of a photodiode photodetector (PD), a transfer gate (TG) for reading out the photogenerated charge onto a floating diffusion (FD), which converts the charge to a voltage. A reset gate (RG) is used to reset the floating diffusion to voltage VDD prior to signal readout from the photodiode. The gate (SF) of a source follower transistor is connected to the floating diffusion for buffering the signal voltage from the floating diffusion. This buffered voltage is connected to a column buss (not shown) at VOUT through a row select transistor gate (RS), used to select the row of pixels to be read out.
As the demand for higher and higher resolution within a given optical format pushes pixel sizes smaller and smaller, it becomes increasingly more difficult to maintain other key performance aspects of the device. In particular, quantum efficiency and cross talk of the pixel start to severely degrade as pixel size is reduced. (Quantum efficiency drops and cross talk between pixels increases.) Cross talk is defined as the ratio of the signal in the non-illuminated to the illuminated pixel(s), and can be expressed as either a fraction or percentage. Therefore, cross talk represents the relative amount of signal that does not get collected by the pixel(s) under which it was generated. Recently, methods have been described to improve quantum efficiency, but at the expense of increased cross talk. (See FIG. 4 in U.S. Pat. No. 6,225,670 B1) Alternatively, vertical-overflow drain (VOD) structures used for blooming protection have been employed which reduce cross talk (S. Inoue et al., “A 3.25 M-pixel APS-C size CMOS Image Sensor,” in Eisoseiho Media Gakkai Gijutsu Hokoku (Technology Report, Image Information Media Association) Eiseigakugiko, vol. 25, no. 28, pp. 37-41, March 2001. ISSN 1342-6893.) at the expense of quantum efficiency.
Increasing the depletion depth of the photodetector will increase the collection efficiency of the device, thereby improving both quantum efficiency and cross talk properties. In the past, this has been achieved by reducing the doping concentration of the bulk material in which the detector is made. However, this approach is known to result in reduced charge capacity and increased dark-current generation (from the increase in the bulk diffusion component) thereby reducing the dynamic range and exposure latitude of the detector.
U.S. Pat. No. 6,297,070 avoids these particular difficulties and describes a photodetector structure wherein the photodiode doping is deeper than other source and drain n-type dopants used in making CMOS image sensors. The increased depletion depth increases the collection efficiency, thereby increasing quantum efficiency while reducing cross talk. When building this structure using high-energy implantation, however, the n-diode to p-epi junction depth (and hence, depletion depth) is limited by the thickness of the masking layer (typically photoresist) that is used to block the implant from other regions of the device. Hence, the maximum resist thickness and aspect ratio (resist height by resist opening) that can be used at this step becomes a limiting factor.
Still further, in the prior art, the n-type region of a pinned photodiode was formed using a single, relatively shallow implant as illustrated by way of example in FIGS. 1b and 1c. The resulting potential profile for such a prior art empty photodiode is shown in FIG. 1d. From this figure, it can be seen that the depletion depth (the point at where the gradient of the electric potential goes to zero) for this example prior-art pixel structure is only about 1.2 um. At green and red wavelengths, the absorption depth in silicon is greater than this depletion depth. Therefore, a carrier generated greater than this depletion depth can diffuse laterally into adjacent photosites which contributes to cross talk.
Therefore, there exists a need within the art to provide a structure that improves both quantum efficiency and cross-talk attributes simultaneously, without impacting other imaging performance characteristics and without the manufacturing issues as described above.